1. Field of the Invention
This invention relates to a disposable syringe, more particularly to a disposable syringe which enables a needle cannula to be retracted into a plunger body.
2. Description of the Related Art
Referring to FIGS. 1 and 2, U.S. Pat. No. 5,211,628 discloses a syringe with an automatic retracting needle, which includes a barrel 11 with a chamber, a flexible lock member 12 fixed in the chamber adjacent to a lower end 111 thereof, a flexible needle seat 13 positioned in the chamber adjacent to the lower end 111 and formed with an opening 131, a needle cannula 14 fastened to the needle seat 13 in the opening 131 and extending through an orifice 112 in the lower end 111, a plunger 15 slidable in the chamber along an axis (X), a shaft 16 disposed in the plunger 15 and having an integral annular lower foot part 161 which is press-fitted against an upwardly extending lip 151 of the plunger 15, and a spring 17 surrounding the shaft 16 and having one end 172 which abuts against an upper end 163 of the shaft 16 and the other end 171 which is retained in a compressed state by engagement of the lip 151 with the lower foot part 161. The lock member 12 has a flexible inwardly protruding lip 121 which bears on a shoulder 134 of the needle seat 13 to prevent the needle seat 13 from moving upwardly.
The shaft 16 further includes a lower protrusion 162 shaped to mate with the opening 131. As such, when the plunger 15 is pressed downwardly along the axis (X), the lower protrusion 162 can be inserted into the opening 131, and the lower foot part 161 spreads the lip 121 outwardly so that the lip 121 is bent to disengage from the shoulder 134. At the same time, the residual force of the lip 121 against the lower foot part 161 causes a slight inward bend of the lower foot part 161 and momentarily causes the lower foot part 161 to lose contact with the lip 151. The compressive force of the spring 17 then urges the upper end 163 of the shaft 16 upwardly. Due to the mating fit of the lower protrusion 162 with the opening 131, the upward movement is also transferred to the needle seat 13 and the needle cannula 14 which are urged upwardly into the plunger 15.
However, in order to ensure smooth retraction of the needle seat 13 and the needle cannula 14 into the plunger 15, the engagement of the lower foot part 161 with the lip 151 cannot be very tight. However, a loose engagement may result in an undesired release of the compressive force of the spring 17. Moreover, some medicine may enter the plunger 15 from a clearance between the lower foot part 161 and the lip 151. Furthermore, the lower foot part 161, which has a conical shape, may interfere with the retraction of the shaft 16 into the plunger 15.